Known automatic transmissions disclosed in U.S. Pat. No. 7,828,688B and U.S. Pat. No. 7,131,926B (hereinafter referred to as Patent references 1 and 2, respectively) include three single pinion type planetary gear mechanisms and six engaging elements including two brakes and four clutches, and establish ten forward speeds and one reverse speed by the engagement of three elements among six engaging elements. As illustrated in FIG. 6, first to third planetary gear mechanisms P11-P13 are arranged in the mentioned order in a row from an input shaft N side to an output shaft T side. Elements that structure the first to third planetary gear mechanisms P11-P13 are referred to as first to third carriers C11-C13 that rotatably support first to third pinions Q11-Q13, respectively, first to third sun gears S11-S13, and first to third ring gears R11-R13, respectively.
The second carrier C12 is connected to the input shaft N. The second ring gear R12 is connected to the third sun gear S13. The second sun gear S12 is selectively fixed to a housing H by a first brake B11 and is selectively connected to the first sun gear S11 by a second clutch CL12. The second carrier C12 is selectively connected to the first sun gear S11 by a first clutch CL11.
The second ring gear R12 is selectively connected to the first carrier C11 by a third clutch CL13. The first ring gear R11 is connected to the third carrier C13. The first carrier C11 is selectively connected to the third ring gear R13 by a fourth clutch CL14. The third ring gear R13 is selectively fixed to the housing H by a second brake B12. The third carrier C13 is connected to the output shaft T.
FIG. 7 shows operation states of clutches CL11 to CL14 and brakes B11 and B12, the operation states corresponding to respective speed stages. A circle provided in FIG. 7 indicates that the element is actuated (the element is in an ON state). FIG. 8 shows a velocity diagram in which the sun gears S11 to S13, the carriers C11 to C13, and the ring gears R11 to R13 that are elements for structuring the first to third planetary gear mechanisms P11-P13 when establishing a reverse speed, or reverse speed stage are arranged with intervals corresponding to gear ratios λ1-λ3 in a lateral axis direction, and rotation speed ratios corresponding to the elements are arranged in a vertical direction.
Further, for example, an automatic transmission disclosed in US2009-0017979A includes a modified construction by an addition of one single pinion type planetary gear mechanism to the automatic transmission for vehicle disclosed in Patent reference 1. That is, the automatic transmission disclosed in Patent reference 3 includes four single pinion type planetary gear mechanisms and six engaging elements including two brakes and four clutches, and establish ten forward speeds and one reverse speed by the engagement of three elements among six engaging elements.
According to an automatic transmission 20 for vehicle disclosed in Patent reference 3 and disclosed in FIG. 9, the same numeral is applied to the component common to the automatic transmission 10 for vehicle of Patent reference 1 shown in FIG. 6, and detailed explanations of the components are not repeated here. As shown in FIG. 9, according to the construction of the automatic transmission 20 for vehicle, a fourth planetary gear mechanism P14 is additionally disposed between the first planetary gear mechanism P11 and the second planetary gear mechanism P12. Hereinafter, each of elements that structures the fourth planetary gear mechanism P14 is referred to as a fourth carrier C14 rotatably supporting a fourth pinion Q14, a fourth sun gear S14, and a fourth ring gear R14.
The fourth sun gear S14 is connected to the second carrier C12. The fourth ring gear R14 is selectively connected to the second ring gear R12 by the third clutch CL13, and is selectively connected to the first ring gear R11 by the first clutch CL11. The fourth carrier C14 is connected to the first carrier C11.
FIG. 10 shows operation states of clutches CL11 to CL 14 and brakes B11 and B12, the operation states corresponding to respective speed stages. A circle provided in FIG. 10 indicates that the element is actuated (the element is in an ON state).
As shown in FIG. 8, according to the automatic transmission 10 for vehicle disclosed in Patent references 1 and 2, when establishing a reverse speed, the first clutch CL11 is operated to be in an ON state so that a rotation drive force of the input shaft N is inputted to the first sun gear S11. Further, drive force for reverse rotation is generated at the first ring gear R11 by the connection of the first carrier C11 and the third ring gear R13 by the actuation of the fourth clutch CL14, and by the fixing of the first carrier C11 by the actuation of the second brake B12. The drive force for reverse rotation of the first ring gear R11 is outputted to the output shaft T via the third carrier C13. That is, according to the automatic transmission 10 for vehicle, the drive force for reverse rotation is generated only by the input to the first planetary gear mechanism P11 and the braking operation for the first planetary gear mechanism P11.
Then, because the third ring gear R13 is fixed and the third carrier C13 rotates in a reverse direction, the third sun gear S13 passively rotates in the reverse direction at speed faster than the third carrier C13. Further, because the second ring gear R12 is connected to the third sun gear S13, the second ring gear R12 rotates in the reverse direction. Because the rotation of the input shaft N is inputted to the second carrier C12, the second sun gear S12 comes to rotate at the high speed. That is, a rotation speed ratio of the second sun gear S12 corresponds to a value H that is calculated by the reduction of an output rotation speed ratio from a value obtained by multiplying the reciprocal of the gear ratio λ2 (=the number of teeth of the second sun gear S12/the number of teeth of the second ring gear R12) and the sum of the output rotation speed ratio and an input rotation speed ratio. Similarly, according to the automatic transmission 20 disclosed in Patent reference 3 (FIG. 9), the second sun gear S12 comes to rotate at the high speed.
Thus, the second clutch CL12 that selectively engages the second sun gear S12 with the first sun gear S11 and the first brake B11 may be susceptible to the generation of the burning because a relative rotation speed is increased. Further, the durability of a bearing or the like that supports the second sun gear S12 declines significantly. In a case where an oil passage is formed at a shaft connected to the second sun gear S12, the durability of a sealing disposed on the shaft may be declined.
A need thus exists for an automatic transmission for a vehicle which is not susceptible to the drawback mentioned above.